1. Field of the Invention
The present invention relates to a solid state imaging device and a method of manufacturing the solid state imaging device, and more particularly to an antireflection film.
2. Description of the Invention
In recent years, in a solid state imaging device, the number of imaging pixels has been increased to gigapixels or more, and a fineness of a pixel region has also been enhanced increasingly. In such a situation, there has been proposed a structure in which a film having a high refractive index for an antireflection is formed on a photoelectric converting portion constituted by a photodiode, while the periphery of a light receiving region is coated with a tungsten shielding film in order to reduce a smear.
In the related-art solid state imaging device, a charge transfer electrode is formed on a surface of a silicon substrate through a gate insulating film and an antireflection film constituted by an insulating film such as a silicon nitride film is formed thereon through an insulating film, and a shielding film having an opening in a light receiving region is formed thereon.
In the solid state imaging device, the antireflection film is to be formed thickly in order to ensure a breakdown voltage between the charge transfer electrode and the shielding film, and the shielding film is formed on the thick antireflection film. With the structure, there is a problem in that a multiple reflection is generated between the shielding film and the antireflection film when the antireflection film is formed thickly and an oblique incident light transmitted from the photodiode into the charge transfer path deteriorates a smear.
In recent years, therefore, there has been proposed a solid state imaging device in which a charge transfer electrode is covered with an ONO film having a three-layer structure including a silicon oxide film, a silicon nitride film and a silicon oxide film in order to ensure a breakdown voltage between the charge transfer electrode and the shielding film and between the silicon substrate and the shielding film (See JP-A-7-45808).
In the related-art solid state imaging device, it is possible to enhance the breakdown voltage by using the ONO film. With the structure, however, the silicon nitride film in the ONO film is removed from the photoelectric converting portion. For this reason, there is still a problem in that an incident light on the photoelectric converting portion is reflected, and a reduction in a sensitivity sometimes causes a problem.
In the case of a two-layer electrode structure, moreover, a silicon nitride film to be an antireflection film for covering a photoelectric converting portion is usually formed on a first electrode. In this case, in FIG. 8 showing a section of a charge transfer electrode portion, there is a problem in that a distance between a shielding film 6 and a second electrode 3b is reduced at an edge of the second electrode 3b and a defective breakdown voltage is apt to be generated. 3a denotes a first electrode, and 4 and 5 denote an insulating film such as silicon oxide. In this connection, a voltage to be applied between the shielding film and a substrate is 0V, while a voltage to be applied between the shielding film 6 and the second electrode 3b is −8 to +15V. Moreover, there is a problem in that a passage for hydrogen at a hydrogen annealing step is eliminated when the silicon nitride is formed to cover the second electrode 3b. There is also a problem in that it is hard to carry out a pattern formation over a concavo-convex surface with high precision when an opening for hydrogen annealing is to be formed on the silicon nitride film.